1. Field of the Invention
The present invention relates to a collimating lens and a collimating system, which condense relatively great intensity light emitted from a light source at an angle onto a projection optical system at a high efficiency, and a projection type image displaying apparatus using the collimating system.
2. Description of the Related Art
FIG. 1 shows a conventional light emitting diode (LED) having a chip 100 as a light emitter, a die 103 which supports the chip 100 and has a reflecting surface 103a that reflects and converges widely diverging light from the light source, and a dome 105 which further condenses the light reflected from the reflecting surface 103a. 
The die 103 and the dome 105 constitute a primary optical system of a projection type image displaying apparatus, which improves the light efficiency by concentrating light in an entrance pupil of a projection type optical system (not shown). However, since light is emitted from the chip 100 at a great divergence angle, the light cannot be entirely concentrated in the projection type optical system, although it is converged by the primary optical system.
LEDs include absorbing substrate (AS) type LEDs, transparent substrate (TS) type LEDs, high-power LEDs, and truncated inverted pyramid (TIP) type LEDs. Among these LEDs, TIP type LEDs are frequently used as a light source for projection type image displaying apparatuses, due to their high light efficiency.
FIG. 2 shows a light intensity distribution with respect to emitting angle variations in a TIP type LED. As shown in FIG. 2, the TIP type LED has an upper emitting angle limit of about 120° and emits peak intensity light at angles of about ±38°. In FIG. 2, the solid line indicates a typical upper bound of the light intensity, and the dashed line indicates a typical lower bound of the light intensity. Since the range of emitting angles of the TIP type LED is too wide, high concentration efficiency cannot be achieved with only the primary optical system.
For this reason, as shown in FIG. 1, a secondary optical system 108 which further concentrates the light from the primary optical system is provided. Conventionally, collimating lenses, such as a biconvex lens as shown in FIG. 3A, a planoconcave lens as shown in FIG. 3B, a biconvex lens as shown in FIG. 3C, and a planoconcave lens as shown in FIG. 3D, have been used as the secondary optical system.
For example, when the biconvex lens is used as the secondary optical system, light is condensed as follows. Referring to FIG. 4A, where a light beam is collimated at a position P1, a light beam at a position P2 greatly diffracts towards the optical axis. In contrast, where a light beam is collimated at the position P2, as shown in FIG. 4B, a light beam at the position P1 slightly diffracts and diverges. As described above, light greatly diffracts at a boundary of a pupil of the biconvex lens 108 whereas light gently diffracts near a center of the pupil of the biconvex lens 108. Due to the difference in diffracting power within the bioconvex lens 108, such conventional collimating lens structures are unable to collimate and entirely concentrate the incident light in the pupil.